Machine-type communication is a communication between two machines each having a corresponding communication device, for example an air interface, A first machine may for example capture information or data from a second machine. Many machine-type communication devices request a transmission of relatively short messages at regular time intervals. For example a beverage vending machine transmits its fill level of beverages via a wireless communication network in regular intervals to a server of a supplier of beverages. The server then informs or indicates the supplier that the beverage vending machine has to be refilled if the fill level of beverages has decreased below a predefined threshold.
The time intervals between subsequent requests of a transmission are in general too long to use or justify a permanent connection for each machine-type communication device, since the data transmission resources have to be shared with other communication devices and are of course limited. On the other hand the time intervals between subsequent requests are not necessarily deterministic: There may be small or large variations in the time intervals between subsequent requests: For example the beverage vending machine sends its fill level to the server of the beverage supplier every two days. However, if a local event, for example a soccer game takes place, a lot of persons may use the beverage vending machine within a relatively short period so that the fill level is lowered to such an extent that refilling is immediately necessary. The beverage vending machine then may request a transmission for its fill level information from a base station in a wireless network on a much shorter timescale to avoid a critically low amount of beverages.
To provide a suitable communication for such data transmission requests a random access channel for example implemented in the 3GPP long term evolution (LTE) was proposed. A corresponding random access channel named bandwidth request channel (BRCH) in IEEE 802.16e/m was also proposed.
However, network traffic intensity in terms of requests of machine-type communication devices is very large in areas with a high population density and a high penetration rate of machine type communication devices. For example a study estimated up to thirty random access channel requests per second and per cell for data transmission in a cell-based network structure for a typical London urban scenario.
The large amount of requests for random access channels per second leads to capacity problems of existing random access channel specifications. These depend on the probability that two or more machine type communication devices use the same time/frequency resources or resource units and in particular the same preamble or CDMA code at the same time. The capacity is defined by the number of requests per second, which may vary in size under the condition that the collision probability is below a predefined value, for example 1%.
Machine type communication devices also coexist with other mobile devices in typical networks, so the amount of resources units for the random access channel cannot be increased arbitrarily.
Since machine-to-machine applications have highly varying quality of service requirements a collision probability of a transmission request by machine type-communication devices is further increased: In case of high a quality of service requirement a machine-to-machine application has to transmit the data for example immediately resulting in an increase of collisions when the penetration rate of machine type communication devices is also very high.